Spectrophotometers generally use a photo-sensor in their optical quantity measuring circuit. The photo-sensor contains in its signal a dark current component and switching noise at the time of change-over of a read-out switch for reading out the output from the sensor, that is, at the time of change-over of the photo-sensor, as noise components.
The noise e.sub.n can be expressed by the following formula: EQU e.sub.n =k.sub.n .multidot.I+e.sub.n ( 1)
where
I: signal quantity,
k.sub.n I: noise component depending upon signal quantity such as change of optical source,
e: noise component not depending upon signal quantity such as switching noise.
Switching noise does not depend upon storage time (exposure time). Therefore, if the storage time is prolonged, the influence of switching noise can be reduced, and S/N (signal-to-noise ratio) can be improved. However, since a photo-sensor can not store a charge above a predetermined level, a suitable storage time exists for an incident optical quantity.
The output of a photosensor falls off on both the long wavelength side and on a short wavelength side. This results from the fact that the sensitivity characteristics of a photo-sensor as well as the incident optical quantity drop on the long wavelength side and on the short wavelength side. In other words, both the sensitivity of the photo-sensor and the incident optical quantity are wavelength dependent.
The S/N ratio deteriorates at these lower output portions from the above-mentioned relation with the noise (because of independence of the signal quantity). As a prior art reference which improves the S/N ratio in a low sensitivity wavelength region, mention can be made of Japanese Patent Laid-Open No. 128823/1982 entitled "Spectrophotometric Instrument" which was laid open on Aug. 10, 1982.
In accordance with this prior art reference, scanning is repeated more times in the low sensitivity wavelength region than in the high sensitivity wavelength region, the spectral output of each wavelength region is added and the spectral output of the full wavelength regions is produced.
However, this prior art instrument involves the following problems:
(1) The measurement time for signals in the low sensitivity regions differs from that of the high sensitivity region; hence, synchronism is lost with the consequent loss of the characteristics determining two spectra.
(2) While measurement is being repeated many times in the low sensitivity ranges, the signals of the high sensitivity regions are discarded, so that the S/N ratio in the high sensitivity region can not be improved.
Turning back again to the formula (1) described already, noise such as the switching noise has a substantially constant value and can be neglected in a region where a signal strength is great. Such noise, however, becomes a critical factor in the low sensitivity regions. Noise components exist also which are proportional to the signal quantity, such as the change of a light source, the temperature change, and the like. For these reasons, the S/N ratio must be improved in the high sensitivity range, too.